Based on the fact that S&P 500 mutual funds track the same index, it could be assumed that any one of the 65 funds is as good as the other, but a recent study by the Investment Company Institute suggests otherwise.

According to the Washington-based trade group, S&P mutual funds are far from one-size-fits-all. Sure, the funds' underlying holdings are practically identical, but creative variations in asset levels, account size, minimum investment requirements and fees differentiate the funds and can have a significant impact on an investor's total return.

And while it's also true that since S&P funds spend next to nothing on research because they track an index, another cost disparity exists due to the fact that some funds charge 12b-1 fees and others don't. And since index fund buyers tend to be bargain shoppers, the S&P 500's product line, which hold a combined $255 billion in assets, are very popular among the budget conscious. Those with expense ratios of 0.40% are garnering more than 90% of the flows, said Sean Collins, chief economist at the ICI, in a report from the Associated Press.